Bill - re-reading Schwing doesn't tell me about differing metals. Only thing I can find (page 43) is that "the trigger plate was blued with the frame, and also head treated. It was fastened to the frame so as to absorb the same amount of blue. Sometimes the metal finishers had difficulty gettng the color to match". After 1939 the bluing process changed, but there is no mention of differing processes or materials between the frame and trigger plate